The invention relates to a method of determining the absolute position of a pointer in an indicating device, the pointer being moved by a rotor of a rotating-field motor via a gearing. The invention relates also to an indicating device having a pointer which is moved by the rotor of a rotating-field motor via a gearing, and having a stop for the pointer, the rotating-field motor having at least 2 field-producing coils which are electrically connected to a control unit which produces a signal at the output for each coil.
In contradistinction to indicating instruments in which the pointer is acted on directly by a rotor in a coil arrangement, as, for instance, in a cross-coil rotating-magnet arrangement, it is advantageous, in certain fields of use, to drive the pointer via a gearing from a rotating-field motor. Depending on the transmission ratio of the gearing, the rotor must then carry out a larger or smaller number of revolutions in order to move the pointer through a given angle. The controlling system knows how many revolutions of the rotor are necessary in order to change the indication by a desired value. In this connection, however, there is the disadvantage that there is no longer a single relationship between the angular position of the rotor in the motor and the position of the pointer. A large number of positions of the pointer are associated with each angular position of the rotor in the motor. The absolute position of the pointer can therefore no longer be determined from the motor data alone.
This is disadvantageous, in particular if the power supply of the controlling system is interrupted for any reason and the controlling system loses the information as to the existing position of the pointer. This can occur in particular when the indicating device is used in automotive vehicles, in which, for instance, upon repair of the electrical equipment, the electrical connection to the battery must be completely interrupted for reasons of safety. The use of external sensors, for instance inductive approximation switches, mechanical limit switches, light barriers or the like, increases the cost of the indicating device. Furthermore, a larger number of structural parts automatically also means a greater weight, which is undesirable, particularly upon use in automobiles or airplanes. Furthermore, the space to house the external sensors is frequently not available.